Our long-term research goals are to understand the mechanisms that regulate stem cell fate decisions. To ensure a life-long supply of blood and immune cells, the hematopoietic stem cell (HSC) must balance self-renewal with differentiation by responding properly to complex and dynamic inputs from the environment. The goal of this proposal is to develop two new mouse models that will serve as important tools for dissecting the intrinsic and extrinsic mechanisms that govern HSC function. These new models will enable us to pursue the mechanisms regulating hematopoietic homeostasis, the role of niches in HSC fate decisions and engraftment, how radiation-induced effects influence these processes, and a range of other questions that cannot be answered by existing methods. The outcomes will facilitate ex vivo HSC expansion and guide strategies for improving the safety and efficiency of HSC transplantation therapies. .